This invention relates to a process for producing a second harmonic wave generating device which converts the wavelength of an incident laser beam to 1/2 of its wavelength by utilizing the non-linear optical effect possessed by an optical crystal, and more particularly to a process for producing a second harmonic wave generating device which can effect phase matching easily and also with extremely high precision during formation of a light waveguide according to the angle matching method.
A second harmonic generation (SHG) device, for example, can be applied to an optical disc memory, CD player, etc. since an output light wavelength becomes 1/2 and thus the recording density can be made to be 4-fold, can be applied to a laser printer, etc. for which acceleration of speed is demanded since light-sensitive sensitivity can be improved through shortening of the wavelength, and can be applied to photolithography, etc. since fine pattern working becomes possible by shortening of the wavelength, and therefore its application fields are very wide.
For such SHG device, heretofore, a bulk single crystal of a non-linear optical crystal with the use of a high power gas laser as the light source has been used. However, in recent years, a semiconductor laser is becoming to be mainly utilized in place of a gas laser for such reasons that (1) there is a strong demand to miniaturize the device as a whole such as an optical disc device, laser printer, etc., (2) while a gas laser requires an external modulator for optical modulation, a semiconductor can be directly modulated, (3) a semiconductor laser is less expensive and easier in handling as compared with a gas laser, etc. Accordingly, it has become necessary to have a device which can obtain high SHG output even by use of a semiconductor laser light source having low output power.
Thus, for obtaining a device having high SHG output with low light source power, it is necessary to confine the laser beam within a narrow region with a light waveguide to make the light power density higher, thereby enhancing the SHG transducing efficiency, simultaneously with effecting phase matching between the incident laser beam and the second harmonic wave.
More specifically, in order to obtain high SHG output, for preventing the second harmonic wave generated and the polarized wave induced through the non-linear optical effect from the fundamental wavelength light from being mutually weakened to be attenuated, it is necessary to make the phase velocity of the both coincident with each other. This corresponds to coincidence of the refractive index of a crystal relative to the fundamental wavelength light with the refractive index relative to the second harmonic wave. However, generally speaking, due to the wavelength dispersion possessed by a crystalline material, the refractive index will vary with wavelength, whereby this demand cannot be satisfied. Accordingly, it becomes necessary to make the refractive indices relative to the two wavelengths by way of some method.
As such method, there the angle phase-matching method and the temperature phase-matching method. The former utilized birefringence of an optical material as shown in FIG. 4, which is a method in which a light with a wavelength of .lambda. is permitted to enter in the direction making an angle .theta. relative to the crystal axis in FIG. 4 so that the refractive indices of the light with the wavelength .lambda. and the light with the wavelength .lambda./2 may be equal to each other. The latter utilizes the change of the refractive index of an optical material with temperature, which is a method in which the refractive indices of the wavelength .lambda. and the light with the wavelength .lambda./2 are made identical.
However, when phase matching based on the angle matching method as described above is performed, precision capable of controlling the angle at about .+-.0.1.degree.is generally required. For this reason, in a slab type light waveguide having a light waveguide formed on the whole substrate surface, the device can be previously cut so that the above angle .theta. may be substantially perpendicular to the incident surface, and subsequently the incident angle of the incident light can be finely controlled to effect phase matching easily. However, in a channel type light waveguide, during preparation of the light waveguide, the light waveguide must be prepared with a precision of about .+-.0.1.degree., whereby there has been involved that an extremely difficult technique is demanded and also a manufacturing device with high precision is required.
On the other hand, when phase matching is effected by use of the temperature matching method, for making a device with high SHG output, the temperature must be controlled at about .+-.0.1.degree. C., whereby it is not practical since a special device has been required in use of the device.
The present invention has been accomplished in order to solve the above problems, and its object is to provide a process for producing a second harmonic wave generating device which can effect phase matching based on the angle matching method easily and at extremely high precision.